RNA enzymes (ribozyrnes) are being developed as treatments for a variety of diseases ranging from inborn metabolic disorders to viral infections and acquired diseases such as cancer. Ribozymes can be used both to down-regulate and to repair pathogenic genes. In some instances, short-term exogenous delivery of stabilized RNA is desirable, but many treatments will require viral-mediated delivery to provide long-term expression of the therapeutic catalyst. Although some variations on naturally occurring ribozymes are available, they have not been very effective in mammalian cells. There is a need to develop modified ribozymes that show improved activity and function in mammalian cells with high efficiency. These ribozymes are useful for developing regulated gene expression systems and have great therapeutic values.